Method and device for determining the respective geometrical position of a body by capacitive sensing

ABSTRACT

For determining the respective geometrical position, the displacement or angle of a body by capacitive sensing, a supply voltage applied to a voltage-distribution element is switched, and/or phase-shifted voltages are applied, in such a way that at least two different distribution patterns of the supply voltage, that follow each other in time, are obtained across the voltage divider.

This is a continuation of application Ser. No. 08/429,250, filed Apr.25, 1995, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a method and to a device including anevaluation circuit which permits capacitive sensing in connection withdisplacement, angle or position sensors so as to enable the position ofmovable bodies to be determined in a no-contact way.

In most of the cases analogue voltage divider circuits are used for thispurpose, which are either designed, according to the known potentiometerprinciple, as displacement encoders, rheostats or potentiometers, oremployed in the form of inductors, for example as differential coils,differential transformers, inductors with short-circuit winding in theform of a tube.

In the case of an analogue resistance potentiometer, a deposit appliedon a substrate by spraying or by vapor deposition serves as a resistancepath on which a wiper slides in contact relation, thereby being put in aposition to pick up different direct voltage potentials of theresistance path, depending on its particular position, and to transmitthem, via a collector wiper connected with it, to a collector path wherethe sensed potential is available for evaluation. Such potentiometers,certain embodiments of which can be used with a very high degree ofprecision as displacement sensors or pick-offs, may under certainconditions lead to problems, due to the constant contact relation whichlatter finally also leads to wear in the case of rapid wiper movements,so that there exists a real demand for a system that allows no-contactsensing of the corresponding measured values.

If a capacitive position sensor or displacement sensor of the kindknown, for example, from DE 28 26 398 C2 is used instead of theinductive measuring systems, which also works in a no-contact way, butinvolves certain measuring inaccuracies and, under certaincircumstances, also non-linearities, then absolutely significantadulterations of the derived measured values must be expected undercertain circumstances, due to the influence of stray capacitances andleakage resistances--a circumstance that is not tolerable in most of thecases.

The capacitive displacement sensor described by DE 28 26 398 C2comprises a pair of obliquely divided, mutually insulated capacitorplates to which an alternating voltage is applied and between which anintermediate plate, serving as a pick-off being adjustable by the lengthof the path to be picked off, is arranged and connected to the input ofan evaluation circuit via a connection cable. The movements of thepick-off lead to constantly varying forces acting on the connectioncable and its connection points; these forces not only result inaccelerated aging of the displacement sensor, but have the additionaleffect, especially due to the changes in the position and displacementof the cable, to cause capacitance variations and varying straycapacitances as well as varying leakage resistances, which constitute adisturbance variable that cannot be specified and, above all, thatcannot be compensated in this way.

The evaluation circuit of this known displacement sensor comprises anoperational amplifier whose one input is connected via the connectioncable to the displaceable intermediate plate, which latter serves aspick-off, and to whose other input the feed-back measuring signal isapplied via a resistor connected to ground. The output of theoperational amplifier is connected, via a rectifier, to other amplifierelements one of which is configured as an emitter follower. As in suchevaluation circuits the arising stray capacitances are in the order ofthe measuring capacitance, and the input resistance of the amplifier isin the range of the sensor impedance, for usual frequencies, a preciseand strictly linear output voltage cannot be expected.

In the case of other capacitive displacement sensors, for which theevaluation circuit is not described at all or only in the form of asimple downstream amplifier (DE 34 41 217), a tightly packed resistancepath, showing a meander-shaped or zigzag configuration, is supported ona substrate surface, and a displaceable pick-off element, being arrangedat a certain distance from that resistance path and designed as a planarannulus, is provided so as to capacitively uncouple the respectivepotential and supply it via a connection line to a measuring circuitconsisting of a voltmeter. The conductor path is, however, connected toa direct-current supply voltage so that capacitive sensing is possibleonly during a displacement performed at a correspondingly high speed,while stationary detection of a position cannot be effected, due to thethen missing measured value. Here again, disturbance variables aresimilarly encountered, due to stray capacitances and leakageresistances, which cannot be eliminated.

Finally, it has been known in connection with a galvanometer to connectthe pointer of the galvanometer, being a movable element, to analternating voltage connection, with the pointer moving in a plane at acertain distance above a resistor element, whereby a voltage drop iscaused in the latter by capacitive coupling, which voltage drop can thenbe evaluated to derive the pointer position (U.S. Pat. No. 3,636,449).

Now, it is an object of the present invention to provide acapacitance-based no-contact position sensor and to design itsevaluation circuit in such a way as to achieve an especially lowsensitivity to disturbances, combined with high measuring accuracy.

Advantages of the Invention

The invention achieves this object and provides the advantage that whilelargely uncritical and low-cost components can be used, the influence ofstray capacitances and leakage resistances can be either fullyeliminated or in any case maintained at a level that does not influencethe desired measuring accuracy in the no-contact measurement ofdisplacements, effected by capacitive coupling.

The invention does not need any costly screening for this purpose as itsucceeds in compensating the influence of stray capacitances to ground,and the input resistance of the circuit, by the automatic control of theoverall gain and/or the supply of the voltage divider circuit, whichpreferably is a usual resistance path of the type also used inpotentiometer-type displacement sensors. The automatic control iseffected in connection with, and altogether tuned to, switching actionsperformed at different points of the evaluation circuit.

Thus, a first preferred embodiment of the invention is based on anarrangement which differentiates between two phases, by timedinterruption of connection lines between the signal source supplying theresistance path, and the resistance path, namely a first phase I inwhich the full voltage then present over the full length of theresistance path--regardless of the position of the capacitive potentialprobe scanning its voltage distribution pattern--is picked up by thatprobe and compared with a reference voltage, and a second measuringphase in which the obtained measured value can be evaluated. Duringphase I the amplification gain of the measured value, that has beenpicked up capacitively by the potential probe, is correspondinglybalanced and/or the signal source feeding the resistance path at acontrollable amplitude is influenced in such a way that any variationsoccurring during the measuring phase II, when one end of the resistancepath is again supplied with the control voltage, are exclusively due tothe respective position of the potential probe, based on itsdisplacement, while any other influences due to stray capacitances orleakage resistances are eliminated because they have been compensatedbefore by the change in gain during phase I, i.e. the phase ofcomparison with the reference voltage. It is of course a preconditionfor this system that the circuitry components involved show a linearbehavior, which condition is fulfilled in the present case.

Such compensation of the interference voltage drop, provoked by straycapacitances and leakage resistances, by automatic control of theoverall gain is also possible when that end of the resistance path thatis connected to ground is supplied with a opposite-phase voltage duringthe measuring phase at the other end, so that in this case, too, avoltage drop occurs across the voltage divider path.

According to another embodiment of the invention it is further possibleto supply the voltage divider with an alternating voltage in such a waythat, alternately, one end is connected to an alternating voltage, whilethe other end is connected to ground. This has the effect that thevoltage appearing at the output represents alternately the voltagedivider ratio or its difference to one. When the gain is then controlledin such a way that the sum of those values corresponds to the referencevoltage, then one obtains two oppositely directed output voltagesrepresentative of the voltage divider ratio, i.e. the desired measuredvalue, and the reference voltage, respectively.

The further features specified in the subclaims permit furtherdevelopments and improvements of the invention.

BRIEF DESCRIPTION OF THE DRAWING

Certain embodiments of the invention will be described hereafter withreference to the drawing in which:

FIG. 1 shows a highly diagrammatic representation of themechanical/constructional design of a preferred capacitive displacementsensor with measuring and coupling capacitor arrangement, as onepossible embodiment of the invention;

FIG. 2 shows a diagram of a potential distribution over a voltagedistribution element, designed as a resistance path, which is scanned bya potential measuring probe;

FIG. 3 shows an illustration of the capacitive relationship between thepotential measuring probe and the resistance path;

FIG. 4 shows a representation of one embodiment of the invention, in theform of a block diagram, where the entire working cycle is subdivided bysuitable voltage control of the resistance path into a comparison andbalancing Phase I and a measuring phase II;

FIG. 5 shows a block diagram of another embodiment of the inventionwhere either both ends of the resistance path are connected to the samevoltage, or only one end is connected to voltage, by means of switchesthat are actuated in accordance with a predetermined timing code;

FIG. 6 shows a block diagram of another embodiment of the invention,where the resistance path is connected to a phase-shifted supplyvoltage;

FIG. 7 shows a voltage/time diagram relating to the circuit of FIG. 6;

FIG. 8 shows a block diagram of one embodiment of the invention wherethe resistance path is supplied with oppositely directed alternatingvoltages, with

FIG. 9 illustrating the two characteristic curves and their cumulativecurve; and

FIGS. 10 and 11 finally show two different approaches for the possiblescreening and electric isolation of certain specific circuitrycomponents.

DESCRIPTION OF THE EMBODIMENTS

The basic idea of the invention is to provide an evaluation circuit fora capacitance-based position sensor that is capable of reliablycompensating the leakage and fault currents occurring in capacitor areasand on conductor paths, and of giving in this way a position sensor ofthis type a degree of accuracy that was not achievable before so that anovel concept of decisive technical importance is achieved incombination with the known advantages of a sensor operating on acapacitive basis, namely no-contact operation, resistance to aging, andthe like. Compensation is effected by switching over the alternatingsupply voltage at the voltage divider element, which latter is scannedby the potential measuring probe and which usually consists of aresistance path, so that in addition to measuring phases comparisonphases and balancing phases are obtained which act with retroactiveeffect either on the alternating voltage supply of the resistanceelement or on amplifiers in the evaluation area so as to eliminate theinfluence of the disturbance variables.

In this connection, a great number of exemplary embodiments arepossible, which will be explained in more detail below, although themechanical/electrical structure of the capacitance-based position sensoris preferably configured in the way that will be described initiallywith reference to the representation of FIG. 1. This description is alsomeant to improve the understanding of the invention, a structure of thetype illustrated diagrammatically in FIG. 1 being especially well suitedto considerably further the desired freeness of the sensor fromdisturbance variables and, insofar, to render the respective evaluationcircuits especially effective.

The capacitance-based position sensor 10, as illustrated in FIG. 1,comprises a potential measuring area 11 and a potential coupling area12, the potential measuring area being, in the case of this embodiment,a real voltage divider element 13 with smooth voltage characteristic,for example and preferably a resistance path of the type commonly usedin potentiometer-based pick-offs (in which case they are supplied withdirect current) or in rotary potentiometers.

The two connections 13a, 13b of the resistance path of the voltagedivider is supplied in this case--via a supply and evaluation circuit 14that will be described in more detail later, in connection with otherembodiments of the invention--with an alternating voltage of constant,maybe also controllable amplitude, and in the case of the illustratedembodiment one of the connections, for example the connection 13b, isconnected to ground so that it can be seen very clearly that adistribution of the alternating voltage dropping in amplitude from theconnection point 13a to the connection point 13b is obtained across theresistance path of the voltage divider.

The resistance path of the voltage divider 13 is of the no-contact type,which means that a potential measuring probe 15 is assigned to it at apredetermined spacing, which thereby capacitively interacts with theresistance path, being thus also capable of tapping the alternatingvoltage potential across the resistance path, that varies across thedistance s (in the measuring direction), the potential measuring probe15, designed in this case as rectangular plate, having an integrating oraveraging effect and tapping at any time that alternating voltageamplitude that results as mean value from the position of the potentialmeasuring probe.

The potential measuring area 11 and the potential coupling area 12 areelectrically isolated one from the other in a suitable way, thepotential coupling area 12 comprising also a movable probe portion,namely a potential coupling probe 17, which moves at a predetermineddistance, i.e. likewise in a no-contact way, across the altogetherelectrically conductive electrode surface 16 of the potential couplingarea 12, in synchronism with the potential measuring probe. Therespective, in this case rectangular, surfaces of the potentialmeasuring probe and the potential coupling probe are connected one tothe other at least electrically, but preferably also through theirmechanical structure, so as to form a common component, for example adouble copper plate arrangement, for being jointly displaced by asuitable support, not show, across the associated surfaces of theresistance path or the coupling electrode surface, in the measuringdirection.

The respective surfaces of the potential measuring probe 15 and thepotential coupling probe 17 form together with the respectivecounter-surfaces of the resistance path and the electrode 16,respectively, capacitors that are designated, in the subsequentevaluation circuits, as measuring capacitor C_(M) and/or as couplingcapacitor C_(K), respectively, and whose capacitance remains almostunchanged across the displacement path so that the generation of themeasured value is not influenced by capacitance variations.

Thus, the potential coupling probe 17 transmits the alternating-currentvoltage amplitude-measuring signal tapped at the potential measuringprobe 15 capacitively and correctly to the electrode surface 16 of thepotential coupling area 12, there being only one additional connection16a which does not change its position and at which the voltageamplitude signal reaches the input 18 of the supply/evaluation circuit14.

The theoretical/physical mechanism is such that when an alternatingcurrent is applied, the potential distribution obtained on the"potentiometer" resistance path according to the representation of FIG.2 can be defined by the following formula:

    Potential ρ=-E·x

or expressed as electric field distribution

    field: E=-gradρ

where,

    ρ1>ρ2>ρ3>ρ4>. . .

is as appears from FIG. 2.

The voltage being an alternating voltage, the potential may berepresented as follows:

ρ=ρo·sin [sin(ω·t)] where

o=amplitude of the potential

ω=angular frequency 2πν

ν=frequency

t=time

The whole system can be perceived as a capacitor composed of a number ofsmall capacitors, as represented in more detail in FIG. 3.

The capacitor equation reads as follows:

    C=εr·εo·A/d,

wherein

C: capacitance

εr: relative dielectric constant

A: capacitor surface

d: distance (of the plates)

The capacitance is defined as follows: ##EQU1##

As to the capacitances, the following is to apply: ##EQU2##

The tapped potential is linearly related to the locus x. →ρ=ρ(x).Through the element capacitances .tbd.Ci, displacement currents I₁ flowinto the probe according to the following formula: ##EQU3## The overalldisplacement current I is then equal to: ##EQU4## which means that theindividual voltage amplitudes are added up to an overall displacementcurrent I.

Accordingly, even the smallest displacements of the plate of thepotential measuring probe are integrated into the tapped alternatingvoltage amplitude as the ratio of the partial voltages naturally changeswhen a displacement by δs takes place. Picking up the measuring signalis effected in a no-contact way, as is the transmission to thestationary electrode surface 16, so that such a position sensor is notsubjected to any mechanical wear and all disturbance variables, such asvariable stray capacitances, which may otherwise be produced by aslittle as bent connection lines, or contact resistances of slidingcontacts, are excluded already at this point.

A first embodiment of one evaluation circuit is illustrated in FIG. 4.The resistance path 20 is connected to an alternating voltage supplysource 21 which has one end connected to ground at 22. The potentialmeasuring probe forms together with the respective associated part ofthe resistance path 20 a first measuring capacitor C_(M) connected inseries with the coupling capacitor C_(K). The circuit diagram of FIG. 4further shows stray capacitors C_(s) connected to ground and theresistance path, respectively, or other supply lines, together with aninput capacitor C_(g) and an input resistor R_(E) at the input of acontrollable-gain amplifier 23, all of which make themselves likewisefelt as disturbance variables.

The output of the amplifier 23 is connected in parallel--via switchesS1, S2, provided in push-pull arrangement, which may of course also takethe form of electronic switches--to a first rectifier 24 whose output isconnected to a controller 25 (for example an operational amplifier), theother input of which is supplied with a reference voltage U_(ref). Inaddition, a storage capacitor 26 is provided at the input of thecontroller 25 for the intermediate storage of the rectifier outputsignal.

The output of the controller 25 is connected to the controllable-gainamplifier 23 and may optionally also act (alone or in combination withsome action on the amplifier 23) on the alternating voltage supplysource 21 so that the latter constitutes a signal source withcontrollable amplitude.

A parallel output branch of the amplifier 23 also comprises a rectifier27, connected in series with a low-pass filter 28, if desired, to whoseoutput the measuring signal (U_(A)) is applied; the switches S1 and S2are synchronously connected to a switch S3 connected into the supplyline to the resistance path 20 in a manner such that the switchingconditions illustrated in FIG. 4 alternate, which means that the switchS3 is open every time the switch S1 is closed, and vice versa; theswitch S2, which connects the measured-value processing elements to theoutput of the amplifier 23, is closed when the alternating supplyvoltage is supplied also to the resistance path 20, with the switch S3in the closed and the switch S1 in the open condition, so that noamplification variations or variations in the amplitude of the supplyvoltage occur at that point in time or during that working phase.

The basic function of such a circuit arrangement, which is capable ofbeing varied in a plurality of ways, is then as follows: During a firstphase, when the switch S3 is open, the switch S2 is open, too, while theswitch S1 is closed--the respective activation of the switches beingtaken care of by the logic and control circuit 29 --, the voltagedivider is supplied by the alternating voltage supply source 21 in sucha way that both ends exhibit the same potential, since with the switchS3 in its open condition no voltage drop occurs across the resistancepath, irrespective of the position occupied at that time by thepotential measuring probe. Consequently, the potential measuring probesenses the same potential at each position, which after having beingamplified by the amplifier 23 and rectified at the input of thecontroller 25--the latter being preferably an I controller--is thencompared with the reference voltage U_(ref). The I controller 25 thenre-adjusts the gain of the amplifier 23 (or influences the amplitude ofthe alternating voltage supply source 21, which is controllable in thiscase) to ensure that an output voltage corresponding to the referencevoltage U_(ref) appears at the output of the rectifier 24, during thecomparison phase during which the output voltage of the measuring probeis independent of the latter's position. At the same time, this signalsensed by the potential measuring probe is during the comparison phaseprovides a measure of the transmission from the resistance path 20 tothe measuring probe and, generally, for the transmission ratio achievedby the evaluation circuit. However, since the operation of the system islinear, this transmission ratio also applies to partial voltages, i.e.at times when the resistance path 20 effectively operates as a voltagedivider.

This is the case during the measuring phase II, when the switch S3 isclosed and, accordingly, the potential measuring probe senses analternating voltage amplitude signal corresponding to its particularposition, so that the described switching-over action, and action on theproperties of the amplifier 23 and/or the amplitude of the supply source21, has the effect to eliminate all influences due to disturbancevariables.

During the measuring phase the derived signal is rectified by therectifier 27, with the switch S2 closed, and transmitted via a low-passfilter 28 to the measuring output where due to the fact that the gainhas been adjusted in such a way that a signal corresponding to thereference voltage U_(ref) will appear under full voltage conditions, thesignal appearing is proportional to the voltage divider ratio, i.e.representative of the position of the measuring probe.

The basic principle explained in connection with FIG. 4, i.e. thesubdivision of the measuring process into a comparison and/or balancingphase and a measuring phase can be implemented in a plurality ofdifferent embodiments. In the case of the embodiment illustrated in FIG.5, both connected ends of the resistance path are supplied withalternating voltages Uv1 and Uv2, respectively, via switches 30, 30',the latter being controlled by a switch sequencing control 31 in such away that either both terminal ends of the resistance path 20' aresupplied with the same voltage, which means that Uv1=Uv2, so that novoltage drop occurs, while during the following (measuring) phase thevoltage is applied only to one end of the resistance path 20', while theother end is connected to ground by the switch control so that a voltagedrop occurs across the voltage divider path, or else--which is in factthe same thing--a voltage is applied to the other end in phaseopposition to the voltage applied to the one end so that in this case,too, a voltage drop occurs across the voltage divider path.

During the first comparison phase I, when no voltage drop occurs, theprobe voltage Us sensed by the probe 33 is transmitted, after havingbeen amplified by the (controllable) amplifier 32 and rectified by thesynchronous rectifier 34, reaches the summation point 36 as directvoltage Ugv, at which point the difference to the reference voltageU_(ref) supplied is derived. The differential direct voltage is thentransmitted to the subsequent controller 37, whose output is eitherapplied--as mentioned before--to the input of the amplifier 33, foradjusting its gain, or is used--alternatively--for adjusting theamplitudes of the voltages Uv1 and Uv2 supplied by the alternatingvoltage generator 38.

Here again, the measured output voltage U_(A), representative of theparticular position of the probe, is transmitted during the measuringphase to the measuring output via the synchronous rectifier 35. Giventhe fact that the gain and/or the supply voltages of the potentiometerpath had been adjusted during the comparison phase in such a way that asignal corresponding to the reference voltage appears under full voltageconditions, the output voltage U_(A) will be proportional to the voltagedivider ratio and the reference voltage.

According to the circuit arrangement illustrated in FIG. 6 it is alsopossible to use phase-shifted supply voltages for the resistance path20', in which case the potential gradients obtained at the differentpoints of the circuit will be as reflected by the diagram of FIG. 7. Inthe representation of FIG. 6 and the following Figures the samereference numerals have been used to identify circuitry componentsidentical to those appearing in FIG. 5, whereas an apostrophe has beenadded to the reference numerals of components with slightly differentfunction.

Due to the shift in phase of the two supply voltages Uv1' and Uv2'generated by the alternating-voltage generator 38, the resistance path20' is supplied with voltages of alternately the same and oppositephase, the voltage Uv2' lagging behind the voltage Uv1' by one fourth ofa period. Consequently, during the periods of time designated by V, thetwo supply voltages have the same, either positive or negative,direction while during the periods of time designated by M they are inphase opposition. Consequently, no voltage drop occurs at the voltagedivider during the "equal phase" periods V, and the electronic switchand the rectifier 34 produce during that period the reference voltageUgv which corresponds to the potential probe voltage Us during thattime. During the periods of time M, the opposite-phase supply voltagesproduce a voltage drop across the resistance path 20', which is used fordetermining the "voltage divider ratio", i.e. the position of thepotential probe (measuring phase II). The measuring voltage Ugm islikewise produced by means of the electronically controlled circuit andrectifier 35, the two values Ugv and Ugm being stored in correspondingcircuits that are implemented in the form of capacitors 26, 26'. Thediagram of FIG. 7 reflects the potential curves for three differentpotential probe positions, namely one near the connection for a supplyvoltage Uv1, one approximately at the middle of the resistance path, andone near the end of the voltage supply point for Uv2.

It goes without saying that similar relationships are obtained when thevoltage divider or a voltage divider element used with similar effect issupplied with square signals at both connections, in which case one endmay be supplied, for example, with a square voltage of a givenfrequency, while the other end is supplied with a square voltage oftwice that frequency. In this case, too, there will be equal-phase andopposite-phase periods of time that may be used as measuring phases andas reference phases, respectively.

Another way to implement the system is that represented by FIG. 8, wherethe resistance path 20' of the voltage divider element is supplied withalternating supply voltages in phase opposition, which means in otherwords that a rising and a dropping characteristic is produced, asrepresented in FIG. 9, by changing the polarity of the potentiometerpath. In this case (see FIG. 8) the alternating voltage source 38'supplies alternating supply voltages Uv1 and Uv2 of equal amplitude,that are alternately applied to the ends of the resistance path 20' viafirst electronic switches 45 and 48, while at any time the respectiveother end of the resistance path 20' is connected to ground via furtherelectronic switches 46 and 47. Consequently, the switches 45 and 47 areactivated in combination with the controlled amplifier circuit 35, whilealternately, during a next phase, the switches 46 and 48 are activatedin combination with the controlled rectifier circuit 34--although noseparate comparison and measuring phases exist in the case of thisembodiment of the invention

The output signals of the two rectifiers 34 and 35, which are controlledin synchronization with the switches 45 to 48, are intermediately storedin the capacitors 26 and 26', and are then transmitted via a summingelement 36' to the controller circuit 37' which is configured as Icontroller preferably to control, here again, the input amplitude of thesupply voltages Uv1, Uv2 to the resistance potentiometer path in such away that the sum of the two output voltages determined remains constant,i.e. that--according to the representation of FIG. 9--the voltage sumis, for example, adjusted to a higher value by comparison with aconstant value, as the sum of the two characteristic curves 1 and 2 musthave a constant value at any position of the path. In the case of theembodiment of FIG. 8, this can again be achieved by effecting acomparison with a reference voltage U_(ref) supplied to the summingelement 36'. The controller 37 then ensures that the sum of the twooutput signals is equal to the reference voltage U_(ref), either byadjustment of the gain of the amplifier 33 or by correspondingre-adjustment of the amplitude of the supply voltage for thepotentiometer, as indicated by the dashed connection line between theoutput of the I controller 37' and the input of the alternating-currentgenerator 38'.

Such a control also has the effect to eliminate the influence of anydisturbance variables due to the properties of the coupling capacitors,it being possible in this case to use the signal of either the one orthe other characteristic, i.e. Uan or Uap, as output voltage.

It has been found to be desirable in the case of such acapacitance-based displacement sensor, angle or position sensor that thevoltage source supplying the voltage divider, and the amplifier inputfor the output voltage should have a defined potential reference pointas otherwise the measurement may become dependent on the potential ratioin the supply voltage and of the sensor housing. It is, therefore,desirable that the ground potential of the supply voltage be connectedto the sensor housing, at least for the frequency range used, asotherwise undefined conditions may arise in which case measuring errorscannot be excluded.

Since, on the other hand, it is not always possible in electric systemsto connect the housing to the ground potential of the supply voltage,certain developments of the invention propose convenient ways foreliminating any disturbances that may arise due to imperfect separationof the potential ratios.

According to the representation of FIG. 10, a first variant comprises aninsulated screening housing, and the voltage divider element, themeasuring probe and the feedback electrode or coupling electrode areenclosed by a screening that serves as ground potential.

The diagram of FIG. 10 shows the voltage divider element 13 aspotentiometer resistance path, the coupling electrode 16 with the commonelement consisting of the potential measuring probe 17 and the potentialcoupling probe 15, the amplifier 33', and a component 41 comprisingthose components that are shown by dash-dotted lines in FIG. 8, andfurther a component 42 comprising the components that are surrounded bydashed lines in FIG. 8.

In the case of such a sensor with insulated screening housing it is ofdecisive importance that no coupling is effected from the housing 40 ofthe sensor to the potential measuring/coupling probe 17, 15 and to theconnection to the amplifier 33'. In addition, it must be prevented thatalternating voltages applied to the resistance path 13 and to the supplylines from the alternating voltage source 42 are coupled into thehousing. To this end, a screening 43 is provided in insulatedarrangement in the housing 40 and connected to the reference potential,for example to the negative supply voltage connection. Conveniently, theentire electronic system, but at least that part in which alternatingvoltages occur, should be arranged inside this screening 43.

Such a solution can be implemented comparatively easily under electricaspects, but may lead to additional costs, due to the spacerequirements, and to constructional difficulties especially where littlespace is available.

Another approach, illustrated in FIG. 11--components identical to thoseused in FIG. 10 are designated by the same reference numerals--,therefore consists in isolating the voltage divider, the preliminaryamplifier and the supply voltage electrically, for example by providingisolating transformers 44, 44' which galvanically isolate the voltagedivider elements and the amplifier as well as the probe signal. It is afurther advantage in this case if the voltage divider is supplied bymeans of one or more transformers so that the supply voltage for theamplifier 33' of the probe signal can be generated also via theadditional block 49.

In the case of this solution, the sensor housing can be used as groundpotential and screening, and in addition such a structure is also morefavorable under electromagnetic compatability aspects, providing at thesame time for a simpler structure.

While the different aspects of the invention have been discussed withreference to block diagrams, it is important to note that certain parts,or even larger circuit units, may of course be implemented by means ofcomponents of a type commonly in use today, especially microprocessors,or the like. Thus, the invention is not limited to the discreet circuitsteps and/or circuit blocks shown; rather, the latter have beendescribed especially for the purpose of illustrating the basic functionsand effects of the invention and certain special functional sequences.It goes without saying that the different components and blocks may beof the analogue, digital or of hybrid kind, or may be implemented asfully or partially integrated units comprising corresponding portions ofprogram-controlled digital systems, such as microprocessors, smallcomputers, digital or analogue logic circuits, or the like. Thedescription of the invention provided above should, therefore, beunderstood as a description of a preferred embodiment meant to explainthe functional and timing sequences and the operation of the respectiveblocks discussed, it being understood that the different components sodiscussed may also be replaced by other ones providing the same effect.

Another aspect of the present invention is the necessity to monitorcertain parts of the circuit or the overall function in order to avoidthat any failure of the sensor, that may remain unnoticed, may causegreater damage. It would, therefore, be convenient to provide ahigher-level circuit, preferably one designed as a microprocessor havinga plurality of inputs, that may be connected to different circuit pointsof the respective embodiments discussed in connection with the Figures,and that initiates an alarm or some other measure when certainpredetermined threshold values are exceeded in upward or downwarddirection.

In addition to failures of the electronic systems, breakage of thesupply line to the sensor, short-circuits of the sensor connections,short-circuits of the measuring electrode and the output voltage have tobe considered as possible failure conditions.

In addition to monitoring the output voltage to ensure that it remainswithin plausible limits, the proper function of the system can beactively verified by other circuit details, namely:

Monitoring the operational gain for the re-adjustment of the referencevoltage;

In addition, to ensure the proper function of the sensor, a minimum anda maximum output signal of a measuring probe must appear so that thecontrol voltage (if a controllable amplifier is provided) or supplyvoltage for the voltage divider can be monitored;

Monitoring the reference comparison, for example by varying therectification by means of control inputs in such a way that, as long asthe system functions properly, the reference voltage appears at theoutput;

Monitoring the reference potential, for example by controlling therectification in such a way that a potential near the referencepotential appears at the output;

Monitoring the opposite-phase output voltage; here again, therectification may be controlled in such a way that the output voltageassumes a value as if the two connections of the voltage divider hadbeen exchanged.

As mentioned before, all these features can be implemented with littlecost by means of correspondingly programmed microprocessors or smallcomputers.

Finally, it should be noted that the claims and, in particular, the mainclaim constitute attempts at formulating the invention, without aprofound knowledge of the state of the art, and that, therefore, theymay not be interpreted as having a prejudicial effect. Consequently, itis understood that all features described and illustrated in thespecification, the claims and the drawings are essential to theinvention and may be incorporated in the claims individually or in anycombination.

We claim:
 1. A method for determining the geometrical position,displacement, or angle of a measuring probe by capacitive sensing,comprising the steps of:guiding the measuring probe along and in spacedrelation to a voltage distribution element to a position between firstand second ends of the voltage distribution element, the voltagedistribution element being connectable to a source of alternatingvoltage and the measuring probe being capacitively coupled to thevoltage distribution element to sense a voltage therebetween; during acomparison phase in which there is no voltage drop across the voltagedistribution element between the first and second ends, comparing themagnitude of a sensed voltage from the measuring probe with themagnitude of a reference voltage value; during the comparison phase,controlling the magnitude of one of the source of alternating voltageand a controllable gain amplifier to make the magnitude of the sensedvoltage equal to the magnitude of the reference voltage value;thereafter, during a measurement phase in which there is a voltage dropacross the voltage distribution element between the first and secondends, sensing a voltage from the measuring probe which corresponds tothe position of the probe between the first and second ends of thevoltage distribution element, the voltage being obtained free ofdisturbance variables.
 2. The method according to claim 1, wherein,during the comparison phase, the source of alternating voltage suppliesthe voltage distribution element with an alternating voltage which isequal at each of the first and second ends so that there is no voltagedrop between the first and second ends.
 3. The method according to claim1, wherein, during the measurement phase, the source of alternatingvoltages is applied to only a first end of the voltage distributionelement while the second end is connected to ground.
 4. The methodaccording to claim 1, wherein, during the measurement phase,opposite-phase voltages from the source of alternating voltage areapplied to the first and second ends of the voltage distributionelement.
 5. The method according to claim 1, including the additionalstep of switching between the comparison phase and the measurementphase.
 6. The method according to claim 1, wherein the measuring probeis connected to a coupling probe to form a single physical unit, the twoprobes traveling the same measuring path and the coupling probe beingcapacitively coupled to a stationary coupling electrode to which ameasured value is delivered.